1. Field of the Invention
The present invention relates to an information processing apparatus, a processing method thereof, and a computer-readable storage medium.
2. Description of the Related Art
In recent years, medical diagnoses and treatments based on moving image capture which uses radiation (for example, X-rays) are being actively conducted. Nowadays, it is especially noteworthy that an X-ray imaging apparatus which uses a flat panel detector (obtained by forming an amorphous silicon TFT and a semiconductor sensor on a glass substrate) is frequently employed. However, a flat panel detector which uses an amorphous silicon TFT cannot amplify a photoelectrically converted signal by pixel by pixel, and therefore reads out a stored charge via a long signal line. Hence, noise is likely to be generated in the image under the influence of external or internal factors. It has recently been found that this noise includes both components with spatial frequencies which overlap those of an object, and components with spatial frequencies which have little overlap with those of the object.
Also, in imaging which uses radiation, a human body must be imaged at a low radiation dose in order to minimize the patient's exposure to the radiation. Hence, a signal with a very small value is read out, and even a slight fluctuation generated in the image is visually perceived. Vertically and horizontally running streaked variations (to be referred to as “line noise” hereinafter), for example, are sensitively perceived by the human eye and therefore exert a large influence on a diagnostic image.
A conventional technique to reduce line noise is a spatial filter, as disclosed in Japanese Patent Laid-Open No. 2003-204955 (to be referred to as “reference 1” hereinafter). In the technique described in reference 1, high-pass filtering is performed for an original image containing line noise in a direction perpendicular to the line noise. After that, low-pass filtering is horizontally performed for the processed image. As a result, a line noise image is obtained and is subtracted from the original image. Thus, the line noise remaining in the image is reduced.
Also, Japanese Patent Laid-Open No. 63-271668 (to be referred to as “reference 2” hereinafter) describes a technique of achieving noise reduction that generates only little blur and residue image by changing the mixture ratio between a spatial filter and a recursive filter in accordance with the degree of motion of the object. Moreover, Japanese Patent Laid-Open No. 60-065679 (to be referred to as “reference 3” hereinafter) describes a method of reducing line noise by calculating the difference between the current signal and a signal delayed by one scanning period, performing threshold processing and suppression processing, and adding the resulting processed signal to the current signal.
In the technique described in reference 1, the line noise undergoes spatial filtering and is thereby reduced. Unfortunately, this technique has only a small effect on line noise with low frequencies in a spatial frequency range nearly equal to that of the frequencies of the object.
Also, in the technique described in reference 2, an image that has undergone two-dimensional spatial filtering undergoes recursive filtering. However, the effect of recursive filtering is suppressed when the object is in motion, while the effect of two-dimensional spatial filtering is relatively great when the object is still. This technique cannot cope with, for example, noise which has spatial frequencies which overlap those of the object and which varies with time. Furthermore, in the technique described in reference 3, a signal delayed by one scanning period in a frame undergoes recursive filtering. Hence, this technique has a great effect on line noise with temporally high frequencies, but only a small effect on line noise with temporally low frequencies.